Furnace fan control



Feb. 6, 1962 A. E. BAAK FURNACE FAN CONTROL 2 Sheets-Sheet 1 Filed sept. s, 1957 Feb. 6, 1962 A. E. BAAK FURNACE FAN CONTROL 2 Sheets-Sheet 2 Filed Sept. 5, 1957 ing the' shut down of the burner.

nited States l arent hice fihl Patented Feb. 6, QSZ

This invention relates to controls for afan or blower in a forced air furnace or the like. Such a furnace will include a heat suorce orV burner located inl a combustion chamber, a` heat exchanger or ductr adjacent the combustion chamber, and a fan or blower or othery air circulating device'for moving air tofbe heated through the heat exchangen i It is an object of the inventionj to provide acontrol whichv will accomplish three results. First, the control should introducen reasonable time delay between the start of the burner` andthe start of the blower to prevent cold air from being blown into the room beingl heated at the beginningof'the-heating-cycle. Second, the control should allow the blower to run afterthe burner is shut down to obtain maximum eficiency from the fuel con sumed and shouldshut offthe blower before theair that is being.. discharged i'nto'the room has reached a temperature whichwill cause the feeling' of ,colddrafh Third, the control'should prevent repeated starting and stopping or short cycling" of' the blower due to residual heat in the heat exchangerA after the burnerisoff'.

In operating'v present day forced` air furnaces, it is often desiredto have the blower start atta' relatively low ternperature,-'for example, the blower may be started when the air in the ductI of the heat exchanger reaches 120 F. However, because of the coldk draft sensation, it will also be desiredtohavethe blower shut on at a relatively high temperature, forex'ample,y 100 F., thereby providing a differential of 20 which would be considered a low differential. Such low kdifferentials have been achieved;y however, such alow differential has produced a problem dur- After the burner is shut olf, the temperature of the air moving through the duct drops fairly'rapidly to the blower shut-oil" temperature andthe blower is shut off. Then the temperature of the air within the' heat exchanger will begin to rise due to residual heat storedin the furnace and after a short period, the low differential between blower-off, blower-on will be passed and the blower'will start. Of course, the temperature of the air in theheat exchanger will immediately begin to drop and the blower will be shut off after a short period. A number of such short cyclesfof blower-on and blower-oft will occur before all of the residual heat is removed from the furnace, producing a quite undesirable sequence ofk operations. Hence, it is seen that in order to prevent short cycling, the differential between blower-start and blower-stop should be high, a` condition ordinarily not compatible with the desired low differential previously discussed. Ac-

` cordingly, it is an object of the inventionvto provide a furnace control which has a low differential during the burner start phase and a high differentiall during the burner stop phase of operation.

It is another object of the invention to provide a furnace control utilizing a thermally responsive element for sensing the air temperature in the heat exchanger, which element may be designed with a high differential and, therefore, have high energy output, and including externally actuated means for biasing the element to a low differential condition when desired.

It is a further object of the invention to provide a furnace control in which the magnitude of the low differential is adjustable and one in which the blower cut-off value is adjustable'. Another object of the invention is to provide such a furnace control' in which the magnitude of the low differentialy is independent of the ambient ternperatiire in which. the control is positioned andi is independent ofthe temperature within the" furnace.

It is another object of the invention toy provide a furnace control utilizinga first bimetal element positioned in theheat exchanger for sensing the air temperature within the furnace to energize andy de-ener'giZe-the blower, and

a second bimetal element positionedv outside ofthe furnace and heated by an independent source actuated' in conjunction with the burner control tobias the first. thermal element toward a low .differential when the burner f control is on and permit the first element to returny to its normal high differential when the burner control is The invention also comprises novell details. of construction and novel combinations and arrangements of parts, which will morefully appear in the course of the following description.y The drawings merely show and the description merely describes-,a preferred embodiment of the present invention which is given by Way ofv illustration or example.

In the drawings:

FIG. l is an isometric view of apreferredembodiment of the invention; k

FIG. 2 is a bottom view taken along the .arrow 2 of FIG. l showing the control1 asmo'untedf in'. a wall o the-furnace,y the furnace being shown in phantom;

FIG. 3y is anexploded view of the delay section of the control; f

FIG. 4 is a schematic view showingv the electrical connections of ra furnace control;

FIG. 5 isv anv enlarged sectional view taken' along; the line 5-5 of FIG. l; f

FG. 6 is a partial sectional view taken along the.y line 6-6 of FIG. 5; and

FIG. 7 is a chart illustrating the operation ofthe control of the invention.

The furnace control of the inventiony illustrated herein includes a switch assembly 10, a furnace thermostat assembly 11, a differential compensation assembly 12, and a mounting plate 13 positioned between the switch assembly lil and the furnace thermostat assembly 1'1. The unit may be mounted on a wall 14 of a furnace (FIG. 2) by clamping the mounting plate 13 against the wall 14 with screws 15, with the furnace thermostat assembly 11 extending into the air duct of the heat eX- changer of the furnace through an opening 16 in the wall thereof. l

VT'ltie switch assembly 10 may be any of several types of snap-action or over-center switches providing for movement between open circuit and closed circuit conditions or positions. One such switch assembly is shown in my Patent No. 2,668,889, entitled Snap Action Thermostatic Switch. In the switch assembly illustrated herein, a housing 19, which is preferably molded of an insulating material, is clamped to themounting plate 13 by screws yZtl with an insulating washer 21 between the housing and the mounting plate. A pair of electrical contacts 22, 23 is mounted in the housing 19, each contact including an insert 24 molded in the housing, a contact button 25 peened over in one end of the insert, and a screw 26 threadedly inserted in the other end of the insert. EX- ternal electrical connection is made tothe switch by placing a wire or lug under the head of the screw and tightening the screw in the insert.

A fulcrurn 29 for ay contact carrier assembly 30 is xed in the housing with a screw 31, the contact carrier assembly 30 having a V-shaped notch 32 for pivotally engaging the fulcrum. A tensionv spring 33 is coupled between a plate 34 and the ylower end (as viewed in FIG.

Vheat to mechanical motion.

3 5) of the contact carrier assembly 30, the plate 34 being supported on a screw 35 fixed in the housing with a nut 36. The tension spring 33 is disposed substantially parallel to the contact carrier assembly and is positioned to move from one side of the pivot poin-t of the carrier assembly to the other side as the carrier assembly moves between the off and on conditions, thereby producing the toggle or overcenter operation of the switch.

The contact carrier assembly 3i) includes a plate 39 having the V-notch 32 therein, an arm 49 and a lever 41 riveted to the plate, both preferably of insulating material, an electrical contact 42 pivotally mounted on the plate, and a spring 43 positioned between the contact 42 and a portion of the plate. The contact 42 serves to close the circuit between the cont-acts 22 and 23 when the contact carrier assembly is in the on or closed circuit condi-tion, the spring 43 serving to equalize the contact pressures. The arm 40 extends from the plate 39 approximately parallel to the mounting plate 13 and the lever 41 projects outward from the housing through openings 44 and 45 in the washer 21 and mounting plate 1 3 respectively. The lever 41 also projects through a sliding dust cover 46 positioned between the washer and mounting plate.

The furnace thermostat assembly 11 includes a triangular U-bracket 49 carried on the mounting plate 13 with the bimetal support bracketS pivotally mounted in openings 51 at the outer end of the bracket 49 (FIG. 2), and an adjusting channel 52 pivotally mounted on the bracket 50 adjacent the openings 51. A pin 53 is fixed in the channel 52 at the end opposite the pivot point thereof, the pin passing through an opening 54 in the bracket 49 and an arcuate opening 55 in a bloweroff temperature control lever 56. The lever 56 is pivoted on the bracket 49 by a rivet 57 and extends through a slot in the mounting plate 13 to the outer face of the switch assembly, the lever being movable between the housing 19 and a shield 58 carried on the mounting plate. A compression spring 61 is positioned between the bracket 50 and the channel 52 for urging these two members apart, the spring being retained in position by a screw 62. The body of the screw freely passes through an opening 63 in the channel and is threadedly engaged with the bracket thus providing an adjustment for the angular relation of the channel and bracket. A bimetal strip 64 is xed at one end to the bracket 51) by rivets 65 and has its other end positioned in an opening 66 in the lever 41, the bimetal strip being apertured, permitting the screw 62 to freely pass therethrough.

The differential compensation assembly 12 includes a time delay circuit which produces a mechanical motion as a function of time, and means for coupling this mechanical motion to the contact carrier assembly in the switch. A preferred form of time delay includes an electric heater and a bimetal element for converting the In the embodiment illustrated herein, an electric heater assembly 79 is fixed in a cover plate 71 by tabs 72 at each end thereof which wrap around the heater assembly, the cover plate being fixed to the switch housing 19 by a screw 73. In the heater assembly, a length of resistance wire is wound on a form 74, the Wound resistor being positioned between two insulators 75 and held in position by a clamp 76. A pair of leads 77 is connected to the ends of the resist- Iance Wire providing for external connection to the heater assembly, the leads being fixed to the control by tabs '78 (FIG. 2) on the shield 58. A bimetal element 79 is fixed to the clamp 76 by rivets Si) and a screw 81 is passed through an opening 82 in the cover plate 71 for threaded engagement with the bimetal element 79 providing adjustment of the angular relation between the bimetal element and the cover plate.

The motion of the bimetal element 79 may be used directly to provide the desiredrdiiferential,compensation in @the control `of the invention.1 However, it is preferred to utilize an additional bimetal element 36 thus permitting compensation for variations in ambient temperature. The bimetal element 36 is fixed to a lever S7 by rivets S3 and the lever is pivotally mounted in the cover plate 71 with tabs 89 positioned in openings 90, the outer ends of the tabs being bent over to maintain the lever in position. When the unit is assembled, the bimetal elements 79 and 86 are positioned parallel to each other and a dimple 91 at the free end of the element 86 provides substantially point contact between the two elements.

A plunger 94 is slidably positioned in an insert 95 which is fixed in the switch housing 19, the plunger having an enlarged head 96 at one end and a cap 97 fixed to the other end thereof for limiting movement of the plunger.

A plate 98 is mounted on the switch housing 19 by a peened over end of the insert 95 (FIG. 6) and a cam 99 is rotatably mounted on the plate 9S with a rivet 100. The variable height surface of the cam 99 is positioned under the cap 97 of the plunger, providing a variable limit on the motion of the plunger to the right (as viewed in FIG. 6).

A typical electrical circuit for use with the furnace control of the invention is shown in FIG. 4. A blower motor 111i is connected to a power source at terminals 111, 112 through a switch 113, this switch corresponding to the contacts 22, 23 and the contact carrier assembly 3) of the invention. A burner control solenoid 114 is connected in series with switches 115, 116, 117 across the secondary of a step-down transformer 113, the primary of the transformer also being connected to the power source at terminals 111, 112. A resistance Winding 119 is connected in parallel with the burner control solenoid 114, this resistance winding corresponding to the resistance winding in the electric heater assembly 70. Switch 115 may be a conventional room thermostat in which the circuit is closed when the temperature of the room being heated is below the desired value. Switch 116 may be the conventional pilot safety switch which is closed when a pilot light is burning. Switch 117 may be the conventional high temperature safety switch positioned in the furnace which is closed when the temperature of the furnace is below the safe operating value.

Consider the operation of the control of the invention when it is installed in a furnace and connected as shown in FIG. 4. When the temperature of the room being heated falls below the value set on the room thermostat, for example, below 70 F., the solenoid 114 and the heating winding 119 will be energized. At this time the burner is started in the combustion chamber of the furnace and the temperature of the air in the duct of the heat exchanger around the furnace thermostat assembly 11 begins to rise. Referring to the graph of FIG. 7, the closing of the room thermostat switch is indicated at t1 and the temperature of the air in the duct of the heat exchanger is indicated by the curve 122. As the temperature in the furnace increases, the bimetal element 64 fiexes and exerts a downward force on the lever 41 (as viewed in FIG. 5, where the switch is shown in the blower-on position). When the bimetal element exerts a sufficient force, the contact carrier assembly will be flipped from the off to the on position. If we eliminate consideration of the plunger 94 for the moment, this switching operation will occur when the furnace temperature is about since the control is ordinarily designed with a high differential. Of course, it can be designed and adjusted to provide nearly any value of differential. When the switch is flipped to the on position, the blower motor is connected to the power source and the heated air in the furnace is circulated through the room being heated.

The furnace will continue to operate in this manner until the temperature at the room thermostat is raised to the desired value, at which time the circuit to the solenoid will be opened by the room thermostat and the burner will be shut off. Burner shut-off is indicated at t3 on the graph ausser/1 of FIG. 7. After the burner is shut oit, the temperature of the air in the furnace drops, causing the bimetal element ferential and permits the use of a high energy bimetal element. The particular shut-oilT value may be adjusted by means of the screw 62 when setting up the control at the factory and by means of the lever 56 after installation of the furnace. v

A major disadvantage of this type of control lies in the fact that the blow is not turned on until the temperature of the air around the bimetal element reaches 170, which point is indicated on the graph or FIG. 7 by t2. The long time between t1 and t2 permits excessive and undesirable heating ofthe furnace structure. This could be eiiminated by operating the bimetal element 64 with a low differential, ie., have the blower go on at 120 and ott at 100, thereby causing the blower to start at time t2. However, the

curing of one problem creates a second problem, namely, short cycling at the shutdown of the burner. After the blower is cut off at f4, the heat stored in the furnace structure will cause the temperature of the air in the duct around the bimetal element to raise sutlicientto switch the blower on again 'if the cut-on temperature is 120, After a short period the temperature will again drop below 100 and the blower will shut off. However, the temperature will again rise causing the blower to come on. When operated with a low diierential of about 20 during the shutdown period, as many as eight or ten such short cycles may occur.

The furnace control `of the invention provides n solution for this problem wherein a high diierentiai bimetal element may be utilized in connection with means for biasing the element to a low differential condition during the start up operation of the furnace. When the burner solenoid is energized at t1, the resistance winding in the heater assembly is also energized. The heat from the winding produces a deflection of the himetal element 79 which, by engagement with the bimetal element 86, produces a pivoting of the lever 87. The lever 87 engages the plunger 94 causing the plunger to engage the arm 40 of the contact carrier assembly (FIG. 6). Thus the con tact carrier assembly is moved toward the on position so that less force is required from the bimetal element 64 via the lever 41 to cause the switch to Hip to the on position. In the example being described herein, the bimetal element in the heater assembly, the lever, and the plunger are selected to reduce the cut-on temperature to 12.0 as shown by the curve 123 of FIG. 7. Of course, this cut-on temperature may be reduced to any desired value within the high diterential range of the bimetal element 64 by suitably dimensioning the components. The cam 99 provides a limit on the travel of the plunger and, therefore, an adjustment for the amount of bias produced. The blower cut-off temperature, as indicated by the curve 124 of FIG. 7 and the adjustments thereof are independent of the operation ofthe diierential compensation assembly, thereby permitting individual adjustments and control of the burner-on and burner-olf temperatures.

When the burner is shut off by the action of the room thermostat, the solenoid and the resistance winding in the heater assembly are de-energized and the temperature at which the control will switch to the on positionfrises to the original value, as seen between the intervals r3 and t4 of FEiG. 7. Thus, at the time the blower cuts oit at fi, the dierential is again high thereby preventing any short cyciing.

Compensation is provided for variations in ambient temperature so that the time delay introduced by the diiierential compensation assembly will not be affected thereby. The Ibimetal elemer s 79 and 86 are substantially identical so that both will move the same distance with variations in ambient temperature thereby maintaining the mechanical coupling constant. The element 79 is intimately clamped to the resistance heater by means of the clamp 76 which preferably is of aluminum or the like having good heat transfer characteristics. The element S6 has mechanicalcontact with the hea-ter assembly only at the dimple 91 thereby making deiiection of the element $6 substantially independent of thefheat generated at the heater assembly, c

Although an exemplary' embodiment of the invention has been disclosed and discussed, it will be understood that other applications of the invention are possible and that y the embodiment disclosed may be subjected to various changes, modifications and substitutions without necessarily departing from the spirit of the invention.

I claim as my invention:

.in an air circulation control' mechanism for a furnace of the like, the combination of: a thermally responsive element adapted to be mounted within a duct and having a fixed portion and a free portion, the position of said free portion being a function or" the temperature of said element; a snap action switch including a first member movable between oit and on positions; means for coupling said free portion to said first member in driving relationship, said member being moved to said on position when the temperature of said element is raised to a first value and being moved to said off position when the temperature of said element is lowered to a second value of less magnitude than said tirst value; a second member movably mounted on said switch for engaging said first member when in said 013i position; a time delay unit adapted to be mounted outside of the duct for engaging said second member to move said rst member from said o position to another o position nearer said on position at a period of time after said time delay unit is energized thereby lowering the magnitude of said first value of temperature; and means for energizing said time delay unit.

References Cited in the tile of this patent UNITED STATES PATENTS 1,583,496 Shafer May 4, 1926 1,885,053 Slough Oct. 25, 1932 2,044,147 Bletz June 16, 1936 2,169,592 King May 30, 1939 2,181,606 Parks Nov. 28, 1939 2,228,515 Foulds Jan. 14, 1941 2,238,219 Fineran Apr. 15, 1941 2,548,983 Klug Apr. 17, 1951 2,642,227 Ray .lune 16, 1953 2,748,223 Frank May 29, 1956 2,786,632 Cunnien Mar. 26, 1957 2,835,448 Page May 20, 1958 

